"Hydro" is weed that has been grown hydroponically in water instead of grown in dirt. "Dro" is just short for "hydro". Usually someone who's growing hydroponically will grow high end strains and give their plants a lot of TLC so the resulting buds are really potent. Because most hydro grown buds are really potent a lot of people have started to use the terms "hydro" or "dro" for any really good weed regardless of how it was grown.